This invention relates to radio receivers and especially to such receivers employing automatic gain control to regulate the output volume for received radio frequency signals of widely varying strength. Such radios provide high gain for low strength radio frequency signals such as are obtained from weak or distant radio stations or from inter-station noise.
In an entertainment band AM radio receiver, the AGC circuitry tends to cause high gain of inter-station noise encountered while tuning from one station to another. Such noise has been silenced in some prior art receivers by the use of muting circuits which switch off the audio signal except when a signal of sufficient strength is received. In addition to being expensive, such circuits provide complete silence between station. Such silence causes the radio operator to lose the "feel" of the radio being on as he is tuning from one station to another and further could fool the operator into believing his radio is inoperative if he turns it on while it is tuned off channel. Further, the abrupt shift from silence to full volume when a station is tuned can be disconcerting if the volume is turned too high. It would be desirable, in such entertainment AM radios, to reduce the volume of inter-station noise to a less objectionable level without removing it completely from the tuning process.
In addition, Citizens band AM radios also use automatic gain control to reduce the difference in volume level of received signals of widely varying strength. Such CB radios also generally include a squelch control which determines the minimum strength of a signal which the receiver is allowed to receive and which may be adjusted by the radio operator. Because of the high noise level of weak, highly amplified signals, a CB radio listener in a vehicle, for instance, might set his squelch level to receive only the strongest signals, even if the channel is not crowded. However, this might cause him to miss an important part of a conversation or a warning of some problem on the read ahead. If apparatus were provided for automatically reducing the volume of weak and noisy signals without losing the signals entirely, the operator could set his squelch level to receive weaker signals. The volume of such weaker signals would, of course, be lower, but they would be received and heard.